Knife sharpener



FIG. 1 is a perspective view of a knife sharpener, showing my new design;

FIG. 2 is another perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a right side view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines depict portions of the knife sharpener in which the design is embodied that for no part of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 